In the art of electrophotography, and particularly xerography, it is well known to coat a conductive substrate, such as an electrically conductive aluminum drum or aluminized polymeric sheeting, with a photoconductive insulating layer to form a composite, layered, imaging article. The surface of the layered imaging structure is then uniformly electrostatically charged and exposed to a pattern of activating electromagnetic radiation, such as light. The charge is selectively dissipated in the illuminated areas of the photoconductive insulator, thus leaving an electrostatic charge image in the nonilluminated areas. The electrostatic charge image can then be developed by a number of means to form a visible image. If desired, the developed image may be fixed or made permanent on the photoconductive insulator surface. Alternatively, the developed image, in the form of electrostatically adhered toner powders or liquids, may be transferred to paper or some other material and subsequently affixed by some suitable means. This may be done, for example, by attracting fusible toner particles to the charged areas, then transferring and fusing the imagewise distributed particles to another surface.
The conductive substrate utilized in such electrophotographic systems usually comprises a metal such as brass, aluminum, gold, platinum, steel or the like and may be of any convenient thickness, rigid or flexible, and in the form of a sheet, web or cylinder. This substrate may also comprise such materials as metallized paper and plastic sheets, conductive polymers, or glass coated with a thin conductive coating. In all cases, it is usually preferred that the support member be strong enough to permit a certain amount of handling. In some instances, an interfacial blocking layer for at least one type of charge carrier is utilized between the base electrode and the photoconductive insulator.
Typical photoconductive insulating materials useful in electrophotography include: (1) inorganic crystalline photoconductors such as cadmium sulfide, cadmium sulfoselenide, cadmium selenide, zinc sulfide, zinc oxide, and mixtures thereof, (2) inorganic photoconductive glasses such as amorphous selenium, selenium alloys, and selenium-aresenic, and (3) organic photoconductors such as phthalocyanine pigments and polyvinyl carbazole with or without additive materials which extend its spectral sensitivity.
The surface potential is of the utmost importance in the development of an electrostatic charge image. For greatest development latitude, the contrast potential (V.sub.c) resulting from different levels of exposure should be as large as possible. The contrast potential (V.sub.c) can be expressed by the equation: EQU V.sub.c =.DELTA..sigma./C I
where .DELTA..sigma. is the change in surface charge density upon exposure to imaging radiation and C is the capacitance per unit area of the photoreceptor.
One prior art method of decreasing C and hence increasing V.sub.c has been to simply increase the photoconductive insulator thickness. However, the low charge carrier mobility in photoconductive insulators used in electrophotographic devices somewhat limits the useful thickness one can employ to decrease C. If the thickness is increased too much, the system will not have a useful discharge speed. In systems where the thickness can be increased somewhat to decrease C, then the increased thickness requirement also restricts the physical characteristics, such as flexibility and adhesion of the photoconductor to the final plate, drum or belt. Thus, to improve potential contrast in such systems, an electrically active transport overlayer on the photoconductor has been used as, for example, in U.S. Pat. No. 3,928,034. For xerographic use, this construction requires that the overlayer be substantially transparent and non-absorbing in the particular imaging radiation wavelength region. In addition, even though the overlayer is substantially transparent, as increasingly thicker layers are required, adsorption and scattering due to included particles and partial crystallization become significant and have a detrimental effect upon the sensitivity of the device and the quality of the copies produced.
The xerographic apparatus disclosed in U.S. Pat. No. 3,684,368 shows the use of photoreceptor constructions which bear some similarities to the constructions of the present invention. The reference shows the use of anodic, porous aluminum oxide layers between the metal layer and photoconductive insulator layer in order to improve the adhesion therebetween. The photoconductive insulative layers tend to be thick to provide decreased capacitance, with the preferred thickness range being 10-15 micrometers. The porous aluminum oxide layer shown in Example 3 is believed to have a thickness of about 0.17 micrometers.
The xerographic photoreceptor shown in Example 3 of U.S. Pat. No. 2,901,348 discloses an aluminum substrate with a 100 Angstrom (approximately 0.01 micrometers) coating of aluminum oxide and a twenty micrometer coating of a vitreous selenium photoconductive insulator layer.